There is a growing need for methods of producing thin polymer films with three-dimensional patterning at scales ranging from several microns to centimeters in industrially relevant quantities. Such patterned polymer thin films are useful, e.g., for optical applications (anti-reflective coatings and filters) and controlled wetting applications (hydrophobic and hydrophilic applications).
There are a number of technologies that currently produce patterned thin films with topologies ranging from several hundred nanometers to microns, including rolling mask lithography (developed by Rolith, Inc. of Pleasanton, Calif., USA), Nanoimprint Lithography (Molecular Imprints of Austin, Tex., USA and Obducat of Lund, Sweden), Holographic Lithography (TelAztec LLC of Burlington, Mass., USA), and Liquid Deposition (“Sharkskin coater” at Fraunhofer Institute in Munich, Germany). All of these methods use master-based methods to create a structure in a photoresist film that is subsequently used to either dynamically create structure in glass, or in thin polymer films that can be embossed and cured using UV light. None of these techniques are able to create arbitrarily varying patterns in a large area format or in a dynamic and digital manner.
Electrohydrodynamic (EHD) patterning is a recently developed technology that involves electrically transferring the micro- or nano-structures formed on a template onto a thin polymer film by shaping the surface of the liquid polymer film through a balance of applied forces on the liquid and the surface tension of the liquid. A surface instability can be driven by van der Waals and thermal forces, but is typically dominated by external forces, if those are present. All external (e.g., electrical or thermal) forces that cause a pressure gradient across the interface can cause this surface instability. The present invention focuses on EHD patterning techniques applied to polymer thin films having a height/thickness that is much less than the length-scale of the instability, so the kinetics of the polymer thin film are completely described by lubrication theory, and the emerging pattern is driven by the fastest growing capillary wave mode. The time scale for generating such polymer thin film EHD patterning is dependent on the liquid polymer's dielectric constant, viscosity, height/thickness and surface tension, the applied voltage (electric field), and the distance between the electrodes or charges used to generate the electric field, and the length scale of the emerging pattern is dependent on the surface tension, and the applied voltage and electric field inside the polymer film. These patterns can either occur at a length scale that is intrinsic to the film properties if the electric field is constant or be forced into specific structures if the electric field is spatially varied. The height of replication for EHD patterning has been limited at the nano-scale in previously proposed techniques because the electric field is sensitive to the spacer height of the template. In addition, conventional EHD patterning techniques are not commercially viable because they are unable to produce commercially useful quantities of film.